The etching of silicon by xenon difluoride is well known. Xenon difluoride requires no external energy sources or ion bombardment to etch silicon, and it exhibits high selectivity to many metals, dielectrics, and polymers used in traditional integrated circuit processing, making it easy to integrate with other processes such as CMOS. One of the first references to the use of xenon difluoride in silicon etching is in H. F. Winters and J. W. Coburn, “The Etching of Silicon with XeF2 Vapor,” Appl. Phys. Lett., vol. 34, no. 1, pp. 70-73, January 1979, where they demonstrate the high selectivity of xenon difluoride to silicon versus silicon dioxide, silicon carbide, and silicon nitride.
The high selectivity of xenon difluoride to silicon is very useful, particularly in the emerging field known as micro-electro-mechanical systems or MEMS. In MEMS, semiconductor based manufacturing technology and processes are used to produce miniature mechanical devices. One example of a miniature mechanical device produced using MEMS technology is the integrated accelerometer described in S. J. Sherman, W. K. Tsang, T. A. Core, D. E. Quinn, “A Low Cost Monolithic Accelerometer,” 1992 Symposium on VLSI Circuits, Digest of Technical Papers, Seattle, Wash., USA, 4-6 Jun. 1992, p. 34-5, which has both a movable mechanical structure and accompanying circuitry to detect the motion of the mechanical structure. The most popular application of this accelerometer is for automotive airbag applications whereby during a crash, the movable mechanical structure moves, and depending on the extent of the motion, the electrical signal produced by the circuitry will determine if the airbag should be deployed. The use of xenon difluoride as an etchant in the production of MEMS devices is well known and is described in, for example, Pister, U.S. Pat. No. 5,726,480.
A number of prior art xenon difluoride etching systems have been described. One example, described in Japanese Patent No. 02187025A, comprises a heated vacuum vessel holding a work piece into which xenon difluoride gas is introduced as the etchant. Another example, shown schematically in FIG. 1, is described in P. B. Chu, J. T. Chen, R. Yeh, G. Lin, J. C. P Huang, B. A. Warneke, and K. S. J. Pister, “Controlled Pulse-Etching with Xenon Difluoride”, Transducers 1997, Chicago Ill., 16-19 Jun. 1997. This system uses a pulsed etching technique, whereby an intermediate chamber, referred to as an expansion chamber, is used to pre-measure a quantity of xenon difluoride gas and to mix the xenon difluoride with other gases, such as nitrogen, to enhance the etching process. The contents in the expansion chamber are then discharged into a main chamber containing the silicon wafer to perform the etching of the silicon. After the xenon difluoride has been sufficiently reacted, the main chamber, and typically the expansion chamber as well, are evacuated through the use of a roughing or vacuum pump. This process is repeated until the desired degree of etching of the silicon has occurred.
The largest drawback of the pulsed etch system described by Chu et al. relates to the cycling nature of the system. Specifically, since the expansion chamber requires time to fill before the etch begins, is open to the main chamber during the etch, and is typically evacuated during the evacuation step of the cycle, it forms a rate-limiting step in the etching process. This limitation, or bottleneck arises primarily from the time it takes to refill the expansion chamber with xenon difluoride gas after the evacuation step of the previous cycle. The waiting time can often be as long as the time of all of the other steps combined and therefore requires the total process time, or the time the wafer spends in the main chamber, to be approximately double the actual etching time. The term overhead is commonly used to refer to the difference between the total process time and the actual etch time.
Yet another example of a xenon difluoride etching system is described in European Patent No. EP 0 878824 A2. This etching system uses a continuous flow of xenon difluoride gas, which is controlled by means of a flow controller in combination with an expansion chamber, also referred to as a reservoir. Although this process does not require the cycling as in the pulsed etching system of Chu, et al., it does tend to waste xenon difluoride since the xenon difluoride gas is constantly flowing and resides in the main chamber only briefly. The relatively expensive nature of xenon difluoride crystals makes this a major concern. Furthermore, these continuous flow systems are much more sensitive to the geometry of the main chamber and to the placement of the xenon difluoride gas inlet hole(s) in the main chamber which may result in eddies in the flow of xenon difluoride gas.
In the MEMS and semiconductor industries, as in most manufacturing industries, throughput in a manufacturing tool is a major concern. Thus, the system described in Chu, et al. may not be attractive to these industries because it has an inherently high overhead. As described in H. F. Winters and J. W. Coburn, “The etching of silicon with XeF2 vapor,” Appl. Phys. Lett., vol. 34, no. 1, pp. 70-73, January 1979, higher etching pressure, that is the pressure of the xenon difluoride gas during the etching process, leads to increased etch rate. Thus, processing time can be decreased and manufacturing throughout can be increased by raising the etching pressure. However, raising the etching pressure in a system such as that described in Chu et al. may not be feasible. FIG. 2 is a graph of the xenon difluoride solid vapor pressure, wherein pressures above the curve at a particular temperature cause the vapor to solidify. As can be seen in FIG. 2, the sublimation pressure of xenon difluoride is approximately 3.8 Torr at room temperature or approximately 20° C. Thus, the pressure in the initial expansion chamber in a system such as that described in Chu et al. is limited to approximately 3.8 Torr if the source of xenon difluoride gas is to be kept at room temperature. Although it is shown in FIG. 2 that heating of the xenon difluoride yields a higher solid vapor equilibrium pressure, heating the xenon difluoride source also accelerates the recrystalization of the xenon difluoride. Ultimately, as the xenon difluoride recrystallizes, its exposed surface area falls, and therefore the sublimation rate of the xenon difluoride from solid to gas falls as well. Since xenon difluoride etching system throughput is based upon etching with xenon difluoride vapor, slower sublimation rates of xenon difluoride vaporhamper the performance of the system.
The ability to accurately determine the etching process end point so as to avoid excess etching is also important. In prior art dry etching processes using xenon difluoride gas, end point detection is typically performed visually. The device being processed is inspected through an optical microscope and etching is stopped when the material being removed is not visible to the eye. Automated end point detection methods using non-optical techniques have not been described for xenon difluoride etching of silicon and related compounds. This is a critical limitation when the process is under full computer control, as found in semiconductor-type cluster tools, and visual inspection is not convenient or possible.
End-point detection systems have been described in the literature for a number of semiconductor manufacturing, etching, and deposition processes, many of which include plasma processing. These have included methods based on optical emission as described in Guinn, et al., U.S. Pat. No. 5,877,032, zero order interferometry as described in Coronel et al., U.S. Pat. No. 5,807,761, RF voltage probing as described in Turner et al., U.S. Pat. No. 5,939,886, acoustic measurements as described in Cadet et al., U.S. Pat. No. 5,877,407, infrared emission measurements as described in Gifford et al., U.S. Pat. No. 5,200,023, atomic spectroscopy as described in Gelernt et al., U.S. Pat. No. 4,415,402, and residual gas analysis as described in Japanese Patent Nos. 11265878, 11204509, and 11145067.